This invention relates to a ferrite core which is adapted for use in a transformer or a choke coil used at a high temperature, and its production method. To be more specific, this invention relates to a ferrite core which exhibits a high saturation flux density at a high temperature of 100xc2x0 C. or higher, and in particular, at a temperature around 150xc2x0 C., and which has high magnetic stability with reduced deterioration in high temperature storage, as well as its production method.
A soft ferrite which is used in producing a magnetic core should have a high saturation flux density and a low power loss. Such ferrite can be used as a ferrite core in a transformer or a choke coil of a DCxe2x80x94DC converter in an EV (electric vehicle) or HEV (hybrid electric vehicle), or as a ferrite core to be placed near the engine of an automobile which will be exposed to a high temperature.
Various properties are required for such soft ferrite core which is used at a high temperature. Exemplary such properties include excellent durability with reduced magnetic deterioration during use at a high temperature, the saturation flux density which experience no significant decrease at a high temperature, and low power loss.
Various proposals have been made to fulfill such requirements. For example, JP-A 10-64715 proposes a magnetic core material of low loss ferrite comprising a MnZnNi ferrite in order to provide a ferrite magnetic core material which exhibits a low loss and a high saturation flux density for a relatively broad frequency band of about 100 kHz to 500 kHz.
The magnetic core material of MnZnNi ferrite disclosed in JP-A 10-64715, however, was still insufficient in the saturation flux density Bs and loss at a high temperature of 100xc2x0 C. or higher, and in particular, at around 150xc2x0 C. as well as in the magnetic stability although it had sufficiently high saturation flux density Bs and sufficiently low loss at 80xc2x0 C.
JP-A 2-83218 also proposes an oxide magnetic material of MnZnNi ferrite. This material has been developed to provide a material which has highly stable magnetic properties, high saturation flux density, and low power loss when used at a high temperature range of 100xc2x0 C. or higher, and in particular, at 100 to 200xc2x0 C. at magnetic field strength (flux density) of 1000 G (100 mT) or higher, and in particular, at 2000 to 5000 G (200 to 500 mT) or higher. In JP-A 2-83218, additives incorporated as auxiliary components are particularly defined. The material disclosed in JP-A 2-83218 exhibits dramatically improved saturation flux density in view of the state of the art at that time. However, there is an increasing demand for improving the properties of the material, and further improvements are required. In addition, despite the effectiveness of the Fe2O3 rich composition of this material in attaining the high saturation flux density, it is quite unlikely that deterioration of the magnetic properties at a high temperature can be effectively avoided as in the case of JP-A 2-83218 in the highly Fe2O3 rich region not tested in JP-A 2-83218 by merely limiting the content of the auxiliary components (additives) to predetermined ranges.
An object of the present invention is to obviate the situation as described above, and to provide a ferrite core which has high saturation flux density Bs at a high temperature of 100xc2x0 C. or higher, and in particular, at around 150xc2x0 C., and which has excellent magnetic stability at a high temperature, experiencing reduced deterioration of magnetic properties, and in particular, reduced core loss at such high temperature (even by trading off some improvement in the level of the loss).
Such an object is achieved by the present invention as defined below.
(1) A ferrite core containing 55 to 59 mol % of iron oxide calculated in terms of Fe2O3, more than 0 to 15 mol % of zinc oxide calculated in terms of ZnO, 2 to 10 mol % of nickel oxide calculated in terms of NiO, and the balance of manganese oxide (MnO) as its main components, wherein
when the main components has a composition represented by the formula:
(Zn2+a, Ni2+b, Mn2+c, Mn3+d, Fe2+e,Fe3+f)O4+xcex4xe2x80x83xe2x80x83(1)
wherein a, b, c, d, e and f meet the relations:
a+b+c+d+e+f=3, and
xcex4=a+b+c+(3/2)d+e+(3/2)fxe2x88x924
the value of xcex4 in formula (1) is such that:
0xe2x89xa6xcex4xe2x89xa62.5xc3x9710xe2x88x923.
(2) A ferrite core according to the above (1) wherein
0xe2x89xa6xcex4xe2x89xa62.0xc3x9710xe2x88x923.
(3) A ferrite core according to the above (1) wherein
0xe2x89xa6xcex4xe2x89xa61.0xc3x9710xe2x88x923.
(4) A ferrite core according to the above (1) wherein
0xe2x89xa6xcex4xe2x89xa60.5xc3x9710xe2x88x923.
(5) A ferrite core according to the above (1) wherein
0 less than xcex4.
(6) A ferrite core according to the above (1) containing 56 to 57 mol % of iron oxide calculated in terms of Fe2O3, 5 to 10 mol % of zinc oxide calculated in terms of ZnO, 3 to 6 mol % of nickel oxide calculated in terms of NiO, and the balance of manganese oxide (MnO) as its main components.
(7) A ferrite core according to the above (1) which has
a saturation flux density at 100xc2x0 C. of at least 430 mT, and a saturation flux density at 150xc2x0 of at least 350 mT when measured by applying a magnetic field of 1000 A/m, and
a core loss at 100xc2x0 C. of up to 1200 kW/m3 when measured by applying a sine-wave AC magnetic field of 100 kHz and 200 mT.
(8) A ferrite core according to the above (1) which has
a saturation flux density at 100xc2x0 C. of at least 450 mT, and a saturation flux density at 150xc2x0 of at least 380 mT when measured by applying a magnetic field of 1000 A/m, and
a core loss at 100xc2x0 C. of up to 900 kW/m3 when measured by applying a sine-wave AC magnetic field of 100 kHz and 200 mT.
(9) A ferrite core according to the above (1) wherein increase in the core loss is up to 4% when stored at 150xc2x0 C. for 2000 hours.
(10) A ferrite core according to the above (1) wherein increase in the core loss is up to 3% when stored at 150xc2x0 C. for 2000 hours.
(11) A ferrite core according to the above (1) wherein increase in the core loss is up to 10% when stored at 175xc2x0 C. for 2000 hours.
(12) A ferrite core according to the above (1) wherein increase in the core loss is up to 50% when stored at 200xc2x0 C. for 2000 hours.
(13) A method for producing the ferrite core of the above (1) comprising the step of firing a molded article, wherein
the firing step comprises heating stage, steady temperature stage, and cooling stage in this order, and
the article is kept in the steady temperature stage at a temperature (steady temperature) of at least 1250xc2x0 C. with the oxygen concentration of the atmosphere kept at 0.05 to 2.0%.
(14) A method for producing the ferrite core according to the above (13) wherein the oxygen concentration of the atmosphere in the steady temperature stage is kept at 0.05 to 0.8%.
(15) A method for producing the ferrite core according to the above (13) wherein the temperature (steady temperature) in the steady temperature stage is up to 1400xc2x0 C.
(16) A method for producing a ferrite core according to the above (13) wherein the cooling stage is accomplished such that,
when a specific temperature in 900 to 1200xc2x0 C. is designated Tn, and when the temperature is reduced from the steady temperature to the temperature Tn,
the oxygen concentration PO2 (unit: %) of the atmosphere at temperature T (unit: K) is either gradually or incrementally reduced to satisfy the relation:
Log(PO2)=axe2x88x92b/T
wherein a is 3 to 14, and b is 5000 to 23000, provided that a and b may or may not alter with the decrease in the temperature T;
when the temperature reaches Tn, the oxygen concentration of the atmosphere is reduced to the level of 0 to 0.01%; and
the temperature is reduced from Tn to the room temperature at a cooling rate which is 2 to 10 times faster than the cooling rate used in the cooling from the steady temperature to the temperature Tn.
(17) A method for producing a ferrite core according to the above (16) wherein the temperature is reduced from the steady temperature to the temperature Tn at a cooling rate of 20 to 200xc2x0 C./hr.
(18) A method for producing a ferrite core according the above (13) wherein, in the temperature range of 900xc2x0 C. to the steady temperature in the heating stage, the oxygen concentration in the atmosphere is maintained to 10% or less, and the heating rate is maintained to 50 to 300xc2x0 C./hr.
Next, the MnZnNi ferrite core of the present invention is described in detail.
The substantial component of the ferrite core of the present invention is constituted by the main components comprising 55 to 59 mol %, and preferably 56 to 57 mol % of iron oxide calculated in terms of Fe2O3, more than 0 to 15 mol %, and preferably 5 to 10mol % of zinc oxide calculated in terms of ZnO, 2 to 10 mol %, and preferably 3 to 6 mol % of nickel oxide calculated in terms of NiO, and the balance of manganese oxide (MnO). In determining the content of various oxides in the main components, the manganese oxide constituting the balance of the main component is calculated in terms of MnO.
When the content of Fe2O3 is too low in the composition as described above, saturation flux density at high temperatures will be reduced. On the other hand, when the Fe2O3 content is too high, improvement of the core loss becomes difficult, and control of the value of xcex4 as described below will be difficult rendering the suppression of the increase in the core loss difficult.
When ZnO is completely absent, decrease in the so called xe2x80x9crelative densityxe2x80x9d is found, and improvement of the core loss becomes difficult. When the ZnO content is too high, saturation flux density at high temperatures tends to become reduced with the decrease in the Curie temperature.
When the NiO content is too low, it becomes difficult to provide the ferrite with a high flux density and low loss at high temperatures. When the NiO content is too high, improvement of the core loss becomes difficult.
The ferrite core of the present invention may further comprise various known auxiliary components in addition to the main components as described above.
Exemplary auxiliary components and their desirable contents are,
SiO2: 0.005 to 0.03 mass %,
CaO: 0.008 to 0.17 mass %,
Nb2O5: 0.005 to 0.03 mass %,
Ta2O5: 0.01 to 0.08 mass %,
V2O5: 0.01 to 0.1 mass %,
ZrO2: 0.005 to 0.03 mass %,
Bi2O3: 0.005 to 0.04 mass %, and
MoO3: 0.005 to 0.04 mass %.
These auxiliary components may be incorporated either alone or in combination of two or more.
Among these, SiO2 and CaO are the most preferred. When the content of SiO2 is less than 0.005 mass %, or the content of the CaO is less than 0.008 mass %, the resulting ferrite will suffer from reduced electric resistance, and hence, increased power loss. When the SiO2 content is in excess of 0.03 mass %, or the CaO content is in excess of 0.17 mass %, abnormal grain growth will take place in the firing, and it will be difficult to obtain the desired saturation flux density Bs and the low power loss.
In the present invention, when the main components of the ferrite core are represented by the ferrite compositional formula (1), below:
(Zn2+a, Ni2+b, Mn2+c, Mn3+d, Fe2+e, Fe3+f)O4+xcex4xe2x80x83xe2x80x83(1)
wherein a, b, c, d, e and f meet the relations:
a+b+c+d+e+f=3, and
xcex4=a+b+c+(3/2)d+e+(3/2)fxe2x88x924
the value of xcex4 (amount of excessive oxygen or cation vacancies) in the formula (1) is such that xcex4xe2x89xa62.5xc3x9710xe2x88x923, preferably xcex4xe2x89xa62.0xc3x9710xe2x88x923, more preferably, xcex4xe2x89xa61.0xc3x9710xe2x88x923, and most preferably xcex4xe2x89xa60.5xc3x9710xe2x88x923.
When the value of xcex4 is too large, it is highly likely that stability of the magnetic properties at high temperatures becomes insufficient, and in particular, increase in the core loss and decrease initial permeability xcexci at a temperature higher than the secondary peak temperature of the ferrite become significant. It is to be noted that the value of xcex4 may be equal to zero. However, when the firing conditions are controlled such that the xcex4 value is zero, it will then be difficult to realize the desired magnetic properties in a stable manner, and the xcex4 value is preferably larger than zero.
The value of xcex4 is calculated from the results of analysis of the composition and quantitative analysis of Fe2+ and Mn3+.
The composition was analyzed by pulverizing MnZnNi ferrite sintered body, and evaluating the MnZnNi ferrite powder with an X-ray fluorescence analyzer (for example, Simultix 3530 manufactured by Rigaku) by glass bead method.
The Fe2+ and Mn3+ were quantitatively analyzed by pulverizing the MnZnNi ferrite sintered body, dissolving the resulting powder in an acid, and thereafter conducting potentiometric titration using K2Cr2O7 solution.
With regard to the Ni2+ and Zn2+, the content was calculated by assuming that all of the Ni and Zn found in the analysis of the composition were present as divalent ions. The amounts of the Fe3+ and Mn2+ were assumed to be the values obtained by subtracting the amounts of Fe2+ and Mn3+ determined by the potentiometric titration from the amounts of the Fe and Mn determined in the analysis of the composition.
The value of xcex4 was calculated by using the values obtained as described above so that the relations:
a+b+c+d+e+f=3, and
xcex4=a+b+c+(3/2)d+e+(3/2)fxe2x88x924
are simultaneously satisfied.
The ferrite core of the present invention is produced by firing the article molded from the powder of starting materials as in the case of conventional ferrite cores. The powder of starting materials may be produced either by calcining the starting materials, or by directly roasting the starting materials with no calcination step.
It might be extremely difficult to completely clarify the conditions required for confining the value of xcex4 within the range as defined above. The inventors of the present invention, however, have confirmed through experiments that the value of xcex4 can be regulated within the range as defined above by adequately controlling the parameters as described below.
(1) Composition of the Main Component
The composition of the main component is preferably limited to the composition as described above.
(2) Firing Conditions
The firing step is preferably accomplished by heating stage, steady temperature stage, and cooling stage which are conducted in this order.
(i) Heating Stage
At the temperature preferably in the range of 900xc2x0 C. to the steady temperature, and more preferably, at the temperature in the range of 600xc2x0 C. to steady temperature, the oxygen concentration of the atmosphere is preferably controlled to 10% or less, and more preferably to 3% or less, and the heating rate is preferably controlled to 50 to 300xc2x0 C./hr, and more preferably to 50 to 150xc2x0 C./hr. The control of heating conditions in the heating stage does not significantly affect to the control of the xcex4 value. The control of the heating conditions, however, results in the production of a compact ferrite core, and hence, in an improved saturation flux density with a reduced core loss.
It is to be noted that, at a temperature lower than the temperature range as specified above, the oxygen concentration may exceed the range as specified above, and may be equivalent to the oxygen concentration in the air.
(ii) Steady Temperature Stage
The temperature is maintained at an adequately selected steady temperature of about 1250 to 1400xc2x0 C. The firing atmosphere used is a relatively oxygen-poor atmosphere which has never been employed in the art, and to be more specific, the firing atmosphere has an oxygen concentration of 0.05 to 2.0%, and preferably 0.05 to 0.8%.
The cooling stage is accomplished such that, when a specific temperature in 900 to 1200xc2x0 C. is designated Tn, and the temperature is reduced from the steady temperature to the temperature Tn, the oxygen concentration of the atmosphere PO2 (unit: %) at temperature T (unit: K) is either gradually or incrementally reduced to satisfy the relation:
Log(PO2)=axe2x88x92b/T,
and when the temperature reaches Tn, the oxygen concentration of the atmosphere is reduced to the level of 0 to 0.01%, and preferably 0 to 0.001%. In the above equation, a is preferably 3 to 14, more preferably 5 to 13, and most preferably 7 to 11; and b is preferably 5000 to 23000, more preferably 8000 to 21000, and most preferably 11000 to 19000.
When the oxygen concentration PO2 is continuously reduced with the decrease in the temperature T, a and b may be typically set at a particular value, respectively. On the other hand, when the oxygen concentration PO2 is incrementally reduced with the decrease in the temperature T, a and/or b may be altered, in the temperature range wherein the PO2 is to be maintained at the constant value, so that axe2x88x92b/T remains at a constant value. In other words, a and b may be either altered in accordance with the decrease in the temperature T, or kept at constant values irrespective of the decrease in the temperature T. When the oxygen concentration is incrementally reduced, the temperature range wherein the oxygen concentration is to be maintained at the constant value preferably does not exceed 100xc2x0 C. When the temperature range wherein the oxygen concentration is to be maintained at the constant value is too broad, the merit of reducing the oxygen concentration with the decrease in the temperature will be less significant. The specific values for a and b may adequately determined to thereby obtain the best results.
The temperature is reduced from the steady temperature to the temperature Tn preferably at a cooling rate of 20 to 200xc2x0 C./hr, and in particular, at 40 to 150xc2x0 C./hr. On the other hand, the temperature is reduced from Tn to the room temperature at a cooling rate which is 2 to 10 times faster than the cooling rate used in the cooling from the steady temperature to the temperature Tn.
The decrease in the oxygen concentration from the steady temperature to the temperature Tn may be accomplished by reducing the ratio of the oxygen gas or the air mixed in the gas other than the oxygen (nitrogen gas, inert gas, or the like), and the ratio of the oxygen gas or the air mixed is typically reduced to zero at temperature Tn. As a matter of fact, the oxygen concentration will not be reduced exactly to zero due to the inevitably remaining or generating oxygen gas even when the ratio of the oxygen gas or air the mixed were reduced to zero. However, the xcex4 value will not be significantly affected by the oxygen remaining at the concentration as low as about 0.01% at the temperature lower than the temperature Tn due to the increased cooling rate in such temperature range.
The temperature Tn may be adequately determined to thereby obtain the best results.
With regard to the atmosphere used in the heating stage, the steady temperature stage and the cooling stage, it is preferable that the gas constituting the atmosphere other than the oxygen substantially comprises nitrogen or an inert gas.
Other conditions which are preferably employed in the present invention are described below in further detail.
The firing temperature (steady temperature) used may be at least 1250xc2x0 C., preferably up to 1400xc2x0 C., and more preferably 1300 to 1360xc2x0 C., and the oxygen concentration used in the steady temperature stage in the firing is as described above. When the firing temperature is less than 1250xc2x0 C., sintering density will be unduly low, and as a consequence, the product will suffer from a low saturation flux density and an increased core loss. On the other hand, an excessively high firing temperature is likely to invite abnormal grain growth and an increased core loss. In addition, when the oxygen concentration in the steady temperature stage in the firing is too high, increase in the core loss during the high temperature storage will be increased. Although the oxygen concentration in the steady temperature stage may be reduced to 0% in view of suppressing the increase of the core loss, it will then be difficult to obtain the desired electromagnetic properties at such an extremely low oxygen concentration in the steady temperature stage, and the core loss will be particularly increased. Therefore, the oxygen concentration is preferably not reduced beyond the range as specified above.
It is to be noted that, in the present invention, the firing time (the time of the steady temperature stage) used may be substantially the same as the one used in the conventional ferrite production process, and most typically 2 to 10 hours. The conditions employed in the steps of calcination, roasting, molding, and the like may also be similar to those employed in the conventional ferrite production process. For example, the pressure used in the molding may be 48 to 196 MPa.
The ferrite core of the present invention exhibits excellent magnetic properties at a high temperature. To be more specific, the ferrite core of the present invention exhibits a saturation flux density at 100xc2x0 C. of 430 mT or more, or 450 mT or more, or even 455 mT or more, a saturation flux density at 150xc2x0 C. of 350 mT or more, or 380 mT or more, or even 385 mT or more when measured by applying a magnetic field of 1000 A/m, and a core loss at 100xc2x0 C. of 1200 kW/m 3 or less, or 900 kW/m3 or less, or even 750 kW/m 3 or less when measured at 100 kHz, 200 mT.
The ferrite core of the present invention also exhibits an increase in the core loss of up to 4%, or up to 3%, or even up to 1% when stored at 150xc2x0 C. for 2000 hours, an increase in the core loss of up to 10%, or even up to 5% when stored at 175xc2x0 C. for 2000 hours, and an increase in the core loss of up to 50%, or up to 40%, or even up to 30% when stored 200xc2x0 C. for 2000 hours.